1. Field of the Invention
The present invention relates to a magneto-optical recording medium on which information is recorded by using a laser beam and a magnetic head.
2. Description of the Related Art
In recent years, magneto-optical recording media have been attracting attention as a type of optical recording medium that allows erasing and re-writing of information.
In conventional magneto-optical recording media, a so-called light modulation system is used in which a magnetic recording layer is locally heated by applying a laser beam thereto, while a magnetic field is also applied, whereby the coercive force of the recording layer is reduced, thereby effecting changes in the orientation of magnetization in the layer, so as to effect the recording of information thereon. In such a system, however, the overwriting of information is impossible, which is an obstacle to the re-writing of information.
To solve this problem, a so-called magnetic-field modulation system has been proposed in, for example, Japanese Patent Laid-Open No. 51-107121, according to which the magnetic field applied to the magnetic recording layer is modulated by an information signal, thereby changing the orientation of magnetization in the recording layer so as to effect the recording of information.
FIG. 9 of the drawings accompanying this specification shows the construction of a conventional magneto-optical recording system. The recording medium generally consists of a substrate 1, a recording layer 6 formed on the substrate 1 and having a magneto-optical recording film (not separately indicated), and a protective layer 7 formed on the recording layer 6. A light source (not shown) is arranged in such a way that a laser beam 92 can be applied to the recording layer 6 through the substrate 1. Further, a magnetic-field generating means 91 (e.g., a flying-type magnetic head) is provided to that side of the recording medium on which the protective layer 7 is arranged.
In a magneto-optical recording medium, and, in particular, in a magneto-optical recording system of the magnetic-field modulation type as described above, the gap between the protective layer of the magneto-optical recording medium and the magnetic head during recording (which gap is indicated at "h" in the example shown in FIG. 9) is set very small (e.g., approximately 5 .mu.m). Thus, to prevent the magnetic head from coming into contact with the protective layer, it is necessary to minimize the surface irregularities of the protective layer. On the other hand, if the surface is too smooth, adsorption may occur between the flying-type magnetic head and the protective layer when they come into contact with each other, resulting in a phenomenon called "head crash". In view of this, a method has been proposed according to which a filler material is mixed in with the matrix material of which the protective layer is made so as to make the surface of the protective layer appropriately rough, thereby preventing such adsorption. For example, Japanese Patent Laid-Open No. 4-64936 discloses a method according to which the problem is solved by adjusting the surface roughness of the protective layer to a level ranging from 0.01 to 0.05 .mu.m. This surface roughness is generally measured in terms of Ra (center line average roughness), which indicates correlativity between the flying stability of the flying-type magnetic head and the probability of adsorption at normal temperature and humidity. However, under a relatively high-temperature/humidity condition, for example, 50.degree. C. and 80%RH, a roughness range of 0.01 to 0.05 .mu.m does not assure flying stability for the head and prevention of adsorption between the head and the protective layer. Thus, even when Ra is within that range, it may be impossible to completely avoid such adsorption under high-temperature/humidity condition.